This invention relates to the use of electromagnetic radiation, particularly millimeter wave radiation, to detect a missing object in a set of objects. More particularly, this invention relates to a millimeter wave or microwave detector for detecting missing packages, such as cigarette packs, in a set of packages, such as a cigarette carton, on a packing machine assembly line.
Modern cigarette making machines are capable of producing upwards of 6,000 cigarettes per minute, wrappinq them in packs of twenty to twenty-five cigarettes, and assembling ten packs into a carton. At those rates, 240-300 packs are assembled into 24-30 cartons each minute. Occasionally, there may be instances when a pack will be omitted from a carton. This may occur, in particular, if one of the packs that is included somehow ends up in a skewed position in the carton, occupying part of the space intended for the missing pack and thereby preventing the missing pack from being included.
It is not commercially acceptable for cigarette cartons to include fewer than the designated number of packs. For this reason, it is necessary to inspect each carton on the assembly line to ensure that each contains ten packs. Known methods of detecting missing packs include beta ray devices which illuminate one side of the carton with beta radiation and examine the radiation exiting the opposite side of the carton. The radiation is partially blocked by the metallic foil or foil/paper laminate which forms part of each cigarette pack. The total amount of radiation exiting the opposite side of a correctly packed carton is known. If the amount of radiation detected is greater than the expected known amount, one can conclude that additional radiation was able to pass through the carton because of a gap where a pack is missing. However, the use of beta ray detectors requires that special care be taken in handling the radioisotopes used to generate the beta rays and invokes government regulations relating to the use of radioactive materials.
Another known type of detector for missing packs is described in commonly-assigned U.S. Pat. No. 4,166,973. That detector employs microwaves at a frequency of approximately 10 GHz, and measures the microwave energy reflected by the foil or foilpaper laminate in the pack. However, the resolution of a microwave detector in that frequency range is not sufficient to see small details associated with some pack orientations that can occur when a pack is missing. In addition, that system uses a complicated single unit for transmission of the microwave energy and for detection of the reflected energy.
Both of the types of deteCtors referred to above would miss certain defects that do not affect the total radiation passed by the carton. For example, because a cigarette pack is only slightly less than twice as tall as it is wide, if a pack is missing and a neighboring pack in the same row turns almost ninety degrees, rotating on an axis normal to its large front and back sides, so that it lies across the space intended for both it and the missing pack, there will be sufficient foil interacting with radiation in that two-pack area to prevent detection of any abnormality by the known apparatus. Smilarly, if a pack is missing, and the neighboring pack in an adjacent row rotates on an axis normal to its longer side faces, so that it lies across both its own space and the neighboring space, there will be sufficient foil interacting with radiation in that two-pack region to prevent detection of any abnormality by the known apparatus.
It would be desirable to be able to provide a missing pack detector which does not use radioisotopes.
It would also be desirable to be able to provide a missing pack detector which would be able to detect more features associated with the various orientations that can be taken by packs of cigarettes in a carton when one or more packs are missing.
It would further be desirable to provide such a detector that does not require the use of complicated specialized apparatus.